"FPGA full Step---Practical Walkthrough" the third chapter of the PCB design inductance, magnetic beads and 0 ohm resistor

2. The difference between inductance, bead and 0 ohm resistor

Inductance: inductor is a energy storage element, many for the Power filter circuit, LC oscillation circuit, low frequency filter circuit, etc., its application frequency is rarely more than 50MHz. For inductance, the inductance value is proportional to the frequency. XL = 2ΠFL to illustrate, wherein the XL is inductance, the unit is Ω, for example an ideal 10mH inductor, at 10KHz, inductance is 628ω, at 100MHz, increased to 6.2mω, so at 100MHz, if a signal through this inductor, will cause the signal quality degradation.

Bead: The material of the bead is iron-magnesium or iron-nickel alloy, these materials have high resistivity and permeability, at high frequency and high impedance, inductance inside the coil between the capacitance value will be minimal. Magnetic beads are usually only suitable for high frequency circuits, so at low frequencies they essentially maintain the integrity of the inductor (including the resistance and reactance characteristic components) and thus cause a micro-loss on the line. At high frequencies, it basically only has a reactive component (JwL), and the resistance component increases as the frequency rises. Like some plls, oscillation circuits, including UHF memory power (Ddr,sdram,rambus, etc.) need to be added to the power input part of the magnetic beads. Ferrite beads can be seen as a series of resistors and inductors, as shown in 3.5, usually used as a passive low-pass filter. At high frequencies, it exhibits resistance, so it can maintain high impedance in a fairly wide frequency range, thus improving the effect of FM filtering. In essence, the bead is a "dissipative device", will convert high-frequency energy into heat, in the performance, often the bead as a resistor to explain, rather than inductance, so the unit of the bead is ohm. Figure 3.6 is the internal structure of the chip bead.

Figure 3.7 is an example calculation process, the power supply voltage rating is 3.3V, the rated current is 300mA, the requirement for 100MHz, 300mVpp noise. After the magnetic bead filter to reach 50mVpp, magnetic beads should be how to choose?

Solution: (1) for the 100MHz signal, 300mV noise needs to be filtered and attenuated to 50mV, which means that the magnetic beads to be divided into 250mV, assuming the load 50Ω,RAC/RL = 250/50, where RL = 50ω, get Rac = 250Ω;

(2) power supply rated current 300mA, derating coefficient according to 0.75, then select the minimum rated current not less than 300ma/0.75= 400mA of magnetic beads;

(3) The IC supply voltage must not be less than 3.0V, then the maximum voltage drop on the DC resistance RDC on the bead should be less than 0.3V, i.e. 300mA * RDC < 0.3V, the RDC < 1Ω.

As described above, Rac = 250Ω (100MHz case), RDC < 1Ω, minimum rated current not less than 400mA. In fact, the selection of magnetic beads, the author also searched for some information, roughly indicating that the selection of magnetic beads if the frequency of noise interference is the best, corresponding to the "Murata" series of magnetic beads to select the model can be. But if the interference source frequency is not known, then if the power supply section, it is recommended to select and input current approximate or exceed the supply current, and then consider the impedance, using the above calculation formula can be. If the signal part, the impedance should be considered first, and then consider the current section.

Fig. 3.5 Equivalent model of magnetic beads

Figure 3.6 Internal structure of chip beads

3.7 Magnetic Beads Calculation

0 Ohm resistor: (1) in the circuit does not have any function, just on the PCB for debugging convenience and compatible design and other reasons.

(2) can be used for jumpers, if a segment of the line does not need to directly subsidize the resistor can be.

(3) When the matching circuit parameters are uncertain, replace with 0ohm, the actual commissioning time, determine the parameters, and then the specific value of the components replaced.

(4) To test the power flow of a circuit, you can remove the 0ohm resistor, connected to the ammeter, so as to facilitate the measurement of power flow.

(5) In the wiring, if the actual cloth is not passed, you can add a 0ohm resistor (should be in-line, should not be surface-mounted).

(6) in the high-frequency signal, acting as inductance or capacitance.

(7) Single point grounding.

(8) between the digital and analog ground between the connection to the common ground, can be connected with 0 ohm resistor, in addition to the 0 ohm resistor equivalent to a very narrow current path, can effectively limit the loop current, so that the noise is limited, the resistance in the band will have attenuation effect, which is stronger than the bead.

"FPGA full Step---Practical Walkthrough" the third chapter of the PCB design inductance, magnetic beads and 0 ohm resistor